Localisation is often implemented by use of radio frequency (RF) communication. This habitually involves the communication of location codes to a central processing device, or local usage of RF signals to locate the geographical position of the device (for example, by triangulation from beacons with known positions). However, in some situations, RF may not be the most appropriate medium. This is because RF spectrum is finite, and thus access thereto is controlled by regulation. Access to many bands in the RF spectrum can involve payment of expensive licence fees to a regulator. Thus, allocation of spectrum for localisation purposes is not always possible. In such circumstances, licence free bands can be defined and used, but these tend to be crowded and may not provide the best means of communication.